In manufacturing substrates like printed circuit boards, various steps are carried out in which copper or copper alloy surfaces must be tightly bonded to a polymeric substrate. In some cases, the required adhesion of the formed bonds must be ensured over a long period. In other cases, a tight bond only has to exist for a short period, e.g. when the polymeric substrate only remains on the copper or copper alloy surfaces during manufacture of the printed circuit board. For example, the tight bond of dry film resists (for structuring conductor lines on printed circuit boards) to the copper surfaces only has to exist while manufacturing the printed circuit board. After the conductor line structures are formed, the resists can be removed.
Another application are solder masks, which are applied to a printed circuit board in order to retain a solderable surface on the board surface. Optimum solder mask resist performance can be only achieved if the printed circuit board surface is properly prepared prior to the application of the solder mask. The solder mask is designed to insulate and protect the copper traces of the printed circuit and keep them from shorting together during wave soldering or reflow soldering. In other words an insulating pattern is applied to a printed circuit board that exposes only the areas to be soldered.
Solder mask options have changed radically in recent years, driven by the demands of surface mount assemblies, as well as environmental concerns, e.g. lead-free solder condition.
With the advent of surface mounting and the introduction of fine pitch components, the requirements for the solder mask application have increased considerably. Due to the increasing complexity of the board circuitry (i.e. finer lines/spaces ratio) and the use of more selective finish techniques, proper adhesion of the solder mask has become a critical issue. Similarly, to withstand the thermal stress encountered during subsequent process steps, better adhesion of the solder mask is required.
Commonly used mechanical pre-treatment processes like mechanical brushing or pumice do not match the desired criteria for optimum solder mask adhesion pre-treatment. Mechanical brushing cannot clean holes and leads to a directional finish often associated with a mechanical deformation. Whereas a pumice pre-treatment can cause residues on the copper surface and makes the rinsing critical in tight geometries.
As a consequence, the need for chemical etching solutions as a pre-treatment step before application of polymeric materials like solder masks has increased during recent years.
Generally, it is a challenge to provide chemical etching solutions to treat copper or copper alloy surfaces which only roughen the surface, but do not excessively etch away the copper layers. Several etching solutions have been developed to meet this need, but exhibit various disadvantages.
Conventional and established treatment methods make use of sodium persulfate or hydrogen peroxide based etching solutions for copper surfaces. These solutions result in uniformly etched the copper surfaces, however only marginally increase the etched surface area. Generally, the lower the increase in surface area is, the poorer the adhesion properties of the subsequently formed bond between the metal and the polymeric substrate are.
Therefore, such conventional etch composition do not yield etch results sufficient for fine line technology. This applies particularly to fine line high-density interconnect (HDI) feature on printed circuit board sizes of 50 μm and today even 25 μm or lower. The sizes particular relate to the L/S value. The L/S value determines the width of a conductor track (L) and the space (S) between two conductor tracks. Fine line applications are typically described by L/S-values of 50 μm or lower for the L and S-value, more particular 25 μm or lower for the L and S-value. In order to enable fine line technology the need for having a resolution with semi aqueous developable solder masks below 125 μm is increasing.
Solder masks applied to such pre-treated copper surfaces show insufficient adhesion to the copper surface.
U.S. Pat. No. 5,807,493 discloses a micro etching solution for copper or copper alloys containing a cupric ion source, an organic acid, a halide ion source and water resulting in adhesion improvement of resins or polymers on copper or copper alloy surfaces.
EP 0855454 B 1 relates to a micro etching solution for copper or copper alloys comprising and oxidizing agent which is a cupric ion or ferric ion source, a polymer compound which contains polyamine chains or cationic group, water and a halide ion source. A micro roughened metal surface is gained by applying this process.
The use of cupric ions like cupric chloride, however, has some disadvantages. The use of cupric ions is expensive and the etch rate achieved is not sufficient for all applications desired for such process. If ferric salts like ferric chloride is used as the etch ingredient, unwanted precipitation and sludge formation of ferric ion compounds is often a problem, particularly when working at high ferric ion concentrations.
Therefore, it is an object of the present invention to provide a method for an efficient pretreatment of copper or copper alloys creating a tight bond between the copper surfaces and polymer surfaces applied thereto and avoiding the disadvantages of the state of the art.
The process should be simple, easy to use, have a high etch rate, be inexpensive and creating no problems in the subsequent processing steps of the substrate, particularly a printed circuit board.
This problem is solved by applying a solution to treat copper or copper alloy surfaces so that a tight bond can be subsequently formed between the copper or copper alloy surfaces and polymeric substrates by contacting the copper surfaces with a solution comprising:                (i) a ferric ion source        (ii) a buffer of an organic acid and an organic acid salt        (iii) a halide ion source        (iv) an accelerator        
The method according to this invention is carried out by contacting the copper or copper alloy surfaces with aforementioned solution. The substrate can be immersed into the solution or the solution can be sprayed onto the copper or copper alloy surface of the substrate. For this purpose common horizontal or vertical equipment can be utilized.
Using a spray, the solution is sprayed onto the substrate having a copper or copper alloy surface at a pressure of 1-10 bar.
For both methods (spray or solution) the process is preferably carried out at a temperature of 20-60° C. The treatment time can vary between 15 and 300 s.
The preferable ranges of the ingredients in the solution are:
Ferric ions1-50g/l, more preferred 3-15 g/lBuffer10-200g/lHalide ion1-100g/lAccelerator0.5-30g/l, more preferred 1-10 g/l
Ferric ion sources can be ferric salts of inorganic acids such as ferric chloride, ferric nitrate, ferric sulfate, ferric bromide. Salts of organic acids such as ferric acetate and solutions such as ferric hydroxide may also be applied. Two or more ferric ion sources may be used together. The amount of the ferric compounds in terms of the content of metal ions is in the range of 1-50 g/l, preferably 3-15 g/l.
The etching solution according to the invention operates at a fairly low ferric ion content. The low ferric ion concentration results in a moderate etch rate, yielding the desired roughness properties while avoiding excessive etching of the copper or copper alloy from the substrate.
Buffer solutions applied in the current invention contain an organic acid and its corresponding salt, preferably its sodium salt. The amount of the acids is preferably in the range of 1-200 μl, more preferred 1-50 g/l, the amount of the corresponding salt preferably in the range of 1-200 g/l, more preferred 1-50 g/l. In order to achieve a good buffer capacity the ratio of the acid to the salt should range between 10:1 to 1:1. Examples for buffer solutions comprise formic acid/sodium formiate, tartaric acid/potassium-sodium tartrate, citric acid/sodium citrate, acetic acid/sodium acetate and oxalic acid/sodium oxalate and mixtures thereof.
The etching solution further contains an accelerator selected from the group consisting of urea, thiourea, guanidinium salts, sulfosalicylic acid, niacinamide and amino acids like glycine, alanine, valine, leucine, iso-leucine and cystein. Furthermore, triazoles, benzotriazoles, imidazoles, benzimidazole, tetrazoles and isocyanates can be used as accelerators.
The accelerators not only increase the etch rate and surface roughness, but also prevent smudge or residue formation on the substrate. The amount of the accelerator preferably ranges between 0.5-30 g/l. The buffer in combination with the accelerator does not only influence the etch rate, but furthermore greatly enhances the surface roughness of the treated copper or copper alloy substrate. It is believed that the accelerator compound absorbs onto the copper surface, thus influencing the etch rate on a sub-micron scale. This causes the imparted roughness to be several times greater than without such accelerator. The accelerator also directs the etching process to the copper or copper alloy grain boundaries, resulting in greatly increased etching on the boundaries compared to the etch rate away from these grain boundaries. This effect further increases the surface roughness achieved, which is much greater than expected for ferric ion solutions. This effect is surprising.
The etching solution according to the present invention results in better roughness values and adhesion properties of the polymeric materials to the copper or copper alloys as for example with those described in EP 0 855 454. This better etching effect is particularly remarkable for the etching of the grain boundaries and so result in deeper crevices in the etched surface.
The halide ions are selected from the group consisting of chloride, bromide and fluoride. Chloride is most preferred. The amount of halide ions added preferably ranges from 0.5-100 μl, more preferred 1-20 g/l.
Optionally, the solution can additionally contain a complexing agent for ferric ions. They are particularly useful when the concentration of the ferric ions in the etching solution is at the higher end of the preferred ranged, i.e. higher than 20 g/l.
The etching solution is free of a polymer compound which contains polyamine chains or a cationic group or both as described in EP 0 855 454, the concentration of which is difficult to control in industrial etching processes.
The chemical composition of the etching solution is maintained at a “steady state” (i.e. approximately constant copper ion concentration) during operation by employing “feed-and-bleed” replenishment. At the beginning of the etch process, the solution is virtually free of copper ions. Upon applying the etch composition, copper ions are dissolved from the metal substrate resulting in an increase in copper ions in the solution. It is necessary to maintain the copper ion concentration constant in order to ensure a reliable etching process of the surface. Therefore, a part of the etching solution is taken out of the process (bleed) and replaced with fresh etching solution (feed). The amount of etching solution to be replenished depends on the copper content and can be calculated according to the following equation:    C (g/l): Cu concentration at steady state    B (in l/m2 Cu surface): bleed rate    D (etch amount of Cu in g/m2): Dissolution of Cu by etch reaction
  B  =      D    C  
The etch reaction is represented by the following equation:Fe(III)Cl3+Cu→Fe(II)Cl2+Cu(I)Cl
The copper ion concentration can be kept constant at a desired concentration by the before mentioned feed-and-bleed technology. In order to make the process cost efficient it is recommended to keep the copper ion concentration between 5 and 60 g/l, preferably 20-40 g/l. At this concentration the copper ions have no negative effect on the etching result.
Since the etching solution of the invention is typically applied to the substrate as a spray, the ferric ions reduced to ferrous ions upon dissolving of the copper will be reoxidised by oxygen from the air. Therefore, the amount of ferric ions to be replenished is equal to the amount of ferric ions which are dragged out resulting in a cheap replenishment.4Fe(II)+O2+4H⊕→4Fe(III)+2H2O
During this reaction protons are consumed in order to oxidize the ferrous (Fe-II) ions back to ferric (Fe-III) ions. The buffer applied in the etching solution maintain the pH-value within the desired range during operation. This ensures a constant etch rate. Generally, the lower the pH-value, the faster the etch rate or copper dissolution rate is. The concentration of the buffer system is adjusted as such that an etch rate appropriate for vertical or horizontal pre-treatment equipments is achieved. Upon choosing the concentrations given before, this aim is achieved. Standard tests (etch rates vs. buffer concentration) may be applied for different systems in order to determine the optimum etch rate. These experiments are common in industry and performed on a routine basis.
After the copper or copper alloy surface has been treated as such, the copper or copper alloy surfaces are rinsed with water, e.g. deionised water and then dried, e.g. with hot air.
Optionally, the etched copper or copper alloy surfaces can also be treated for 5-300 seconds with diluted acid after being rinsed, preferably with 10 weight % hydrochloric acid. After being treated with acid, the copper surfaces are again rinsed, preferably with deionised water.
The samples are preferably treated by spraying the etching solution according to the invention onto the samples. The solution can be sprayed in a vertical mode or horizontal mode, depending on the equipment desired. Alternatively, the samples can be immersed into the etching solutions. To achieve the same roughness values compared to spraying, the solution needs to be penetrated by oxygen, e.g. by bubbling air through it.
The etching solution of the invention also has the advantage to be compatible with commonly used solder masks and selective finish techniques, e.g. hot air solder level (HASL), immersion silver, electroless nickel gold (ENIG) and immersion tin.
To illustrate the broad scope of this invention, ENIG and immersion tin are selected for sample preparation as selective finishes. The immersion of printed circuit boards into an electroless nickel gold process requires and excellent adhesion of the solder mask on the copper surface due to the relatively long treatment times and high process temperatures.
The inevitable hydrogen evolution is also critical for solder mask adhesion. When applying immersion tin baths at higher temperatures, which generally contain thiourea and acids, attack of the bond between the solder mask and the copper surface is likely to occur. The better the adhesion properties between the solder mask and the copper, the less likely such an attack is.